rounded square in openscad

rounded_corner.scad

module rounded_square( width, radius_corner ) {
	translate( [ radius_corner, radius_corner, 0 ] )
		minkowski() {
			square( width - 2 * radius_corner );
			circle( radius_corner );
		}
}

module alternative_square( width, r_c ) {
	hull() {
		translate( [r_c, r_c, 0] ) circle( r_c );
		translate( [r_c, width - r_c, 0 ] ) circle( r_c );
		translate( [width - r_c, r_c, 0] ) circle( r_c );
		translate( [width - r_c, width - r_c, 0] ) circle( r_c );
	}
}

Can be simplified and add a feature at the same time:

module rounded_rect(l, w, r)
{
    translate([r, r, 0])
    offset(r=r, $fn = 60) 
    square([l-(2*4), w-(2*r)]);
}

@jcoehoorn I guess you have a typo (l-(2*4)) in there.
The following produces the correct length for me:

module rounded_rect(l, w, r)
{
    translate([r, r, 0])
    offset(r=r, $fn = 60) 
    square([l-(2*r), w-(2*r)]);
}